If you remember an early version of Quake (may have been the first Quake), You started out in a hall and there were three
different paths you could take. If you walked one way, the difficulty was easy, another way hard, etc.
When I was making the game "Below", I did something similar. I used three different paths you could take. The paths were marked with huds to show the difficulty. Just beyond the entrance, the paths joined together again, but in each path was a different trigger zone. The different zones controlled things like starting health, which weapons got spawned, how many enemies were spawned and how strong they were, etc, thereby creating different levels of difficulty.
That's one way. I would think that you could do the same with a keystroke selection and some imaginative scripting.
Brian.
!retupmoc eht ni deppart m'I !pleH